Turbulence inducing electrogasdynamic precipitator

ABSTRACT

An electrostatic precipitator in which particles entrained in the gas stream are charged electrically by a corona discharge field established in opposition to the gas stream between a plurality of emitter electrodes and a screen attractor electrode upstream of an surrounding the emitter electrodes, inducing turbulence in the stream. The velocity at which the particles pass through the charging field is increased by a dielectric converging-diverging nozzle located across the gas stream closely adjacent the particle charging field to retard migration of the charged particles to the screen attractor. The charged particles are collected downstream of the discharge field on an electrically charged moving dielectric member by which an electric precipitation field terminating at a passive reference electrode is maintained across the gas steam. Charges and charged particles are held on the collection surface of the dielectric member by an electric field created between that surface and a conductive member adjacent the opposite surface of the dielectric member. At a location isolated from the gas stream, the collected particles are removed from the collecting surface.

llit States Patent Meredith C. Gourdine West Orange;

Geza Von Voros, Glen Rock; Ta Kuan Chiang, Berkeley Heights, all of, NJ.

[72] Inventors [2]] Appl. No. 814,676

[22] Filed Ap'r. 9,1969

[45] Patented June 1, 1971 [73] Assignee Gout-dine Systems, Inc.

Livingston, NJ.

[54] TURBULENCE INDUCING ELECTROGASDYNAMIC PRECIPITATOR 20 Claims, 9 Drawing Figs.

[52] U.S.Cl 55/114,

55/129, 55/138, 55/145, 55/149, 55/151, 55/155 [51] Int.Cl B031: 3/10 [50] Field ofSearcll 55/113,

Primary ExaminerDennis E. Talbert, Jr. Attorney-Brumbaugh, Graves, Donohue & Raymond ABSTRACT: An electrostatic precipitator in which particles entrained in the gas stream are charged electrically by a corona discharge field established in opposition to the gas stream between a plurality of emitter electrodes and a screen attractor electrode upstream of an surrounding the emitter electrodes, inducing turbulence in the stream. The velocity at which the particles pass through the charging field is increased by a dielectric converging-diverging nozzle located across the gas stream closely adjacent the particle charging field to retard migration of the charged particles to the screen attractor. The charged particles are collected downstream of the discharge field on an electrically charged moving dielectric member by which an electric precipitation field terminating at a passive reference electrode is maintained across the gas steam. Charges and charged particles are held on the collection surface of the dielectric member by an electric field created between that surface and a conductive member adjacent the opposite surface of the dielectric member. At a location isolated from the gas stream, the collected particles are removed from the collecting surface.

? GAS FLOW PATENTEDW K197i 3,581,468

5 UZM' m2 2m 2M GEZA VON VOROS 8| B TA KUAN CHIANG MEREDITH C. GOURDINE,

TURBULIENCE INDUCING ELECTROGASDYNAMIC PRECIPITATOR BACKGROUND OF THE INVENTION The present invention relates to the electrostatic precipitation of particles entrained in a gas stream and, in particular, to apparatus for more efficiently precipitating particles of diverse sizes by electrostatically generating turbulence in the gas stream and by collecting the particles on a moving collection surface.

Electrostatic precipitation of dust particles, such as the fly ash entrained in industrial exhaust gases, is widely known and used throughout many industries. Typically, electrostatic precipitation in the prior art systems is carried out in.

precipitators in which the dust is precipitated on a-plurality of fixed collection plates disposed along a gas flow channel. In many installations, and particularly where heavy dust loading (i.e., particle concentration) exists, or where a full range of particle sizes is encountered, the prior art systems do not efficiently remove the dust from the gas stream. Poor efficiency is attributable partly to saturation of the surface areas of the collection plates with precipitated dust, with the result that the charging or precipitation fields, or both, are quenched to the extent that further particle collection is retarded. In addition, the prior art precipitators are less efficient collections of small particles (particles of less than 1 micron diameter) than they are of larger particles because of inherent limitations. Generally, small particles are collected less efficiently because they acquire an electrical charge at a much slower rate when subjected to a corona discharge than do larger particles. As a consequence, the mobility of small particles is several orders of magnitude less than that of larger particles and, accordingly, their drift speeds in an electric precipitation field are correspondingly less. It often occurs, therefore, that small particles are passed through conventional precipitators and are exhausted into the atmosphere along with the gas stream.

Moreover, the cleaning of the collection plates of the prior art systems presents a serious problem inasmuch as a substantial amount of reentrainment of the dust occurs. Removal of the dust from the plates is normally accomplished by vibrating the collection plates in situ to dislodge dust particles which fall by gravity into hoppers disposed beneath the plates. Because of the proximity of the plates to the gas flow channel, however, some of the dislodged dust particles are reintroduced into the gas stream. The reentrained particles must be recharged and again collected for effective removal from thegas stream. This, of course, necessitates a lengthening of the collection zone to compensate for reentrainment of the particles during the removal operation.

Prior attempts to increase the efficiency of the prior art systems have included the use of additional collection plates'in the gas flow channel to increase the surface area available for deposition of the dust. Although producing some increase'in efficiency and length of operating times between cleanings, this approach results in significant increases in the overall size and complexity ofthe precipitation systems and is therefore not entirely satisfactory.

Precipitators have also been proposed in which the particle collection surfaces are in the form of endless belts that continuously move through the collection zone. For example, in the U.S. Pat. No. 2,579,440 to Palmer, dust particles are collected on a moving belt that has been charged to a polarity opposite to that imparted to the dust particles and are removed from the belt surfaces at a point remote from the gas stream. Such apparatus is, however, subject to the disadvantage that the charges applied to the surfaces of the belts are susceptible of being pulled off the belts by the charged dust particles. When this occurs, the particular dust particles affected are not removed from the gas stream and, in addition, the precipitation field is weakened because the charge carried by the belts is reduced.

A further difficulty associated with this type apparatus is that the particles are subject to reentrainment, notwithstanding that they are removed from the belts at a quiescent location. Upon contacting the collection surface, the particles are either neutralized or reversed in polarity depending upon the residual strength of the charges imparted to the belts. In either event, they are susceptible to being dislodged from the belt surfaces by the motion of the gas stream, or by the attractive forces of the oppositely charged dust carried by the gas stream, because they are retained on the belts primarily by nonelectrical effects.

In the past, efforts have been made to maintain streamlined or relatively smooth flow contours within the collecting sections of electrostatic precipitators in order to keep turbulence to a minimum and thereby prevent reentrainment by aerodynamic forces of particles lodged on the collecting surfaces. It has been found, however, that turbulence actually aids the. rate of particle precipitation to the collecting surfaces by the action of the turbulent currents in carrying particles to the gas stream boundary. Therefore, turbulence can be gainfully utilized to increase precipitation efficiency, provided that particles reaching the gasstream boundary are subjected to sufficient electrostatic forces to prevent them from being swept again into the gas stream. It is one object of this invention to enhance electrostatic precipitation efficiency by combining the beneficial effects of turbulence with an improved collection section in which the electrostatic field forces acting on the deposited particles are strong enough throughout the section to capture and retain most ofthe particles reaching the gas stream boundary.

SUMMARY OF THE INVENTION In accordance. with the invention, a moving dielectric surface member is heavily charged electrostatically, with the electrostatic charge being retained on the dielectric member by a strong electric field terminating on an electrode adjacent to the dielectric member but located externally of the gas stream. Particles entrained in the gas stream entering the precipitator are charged electrically to a polarity opposite to the charged polarity of-the dielectric member. Charging of the particles is accomplished preferably in a corona discharge having a substantial component in opposition to the gas flow to generate controlled turbulence in the stream. The charged surface of the dielectric member, together with the space charge field in the flow, establishes an extremely strong electric field normal to the moving surface to react with the charged particles and move them into contact with the moving surface. The electric field between the charged surface and;

the exterior electrode has a value exceeding that of the electric precipitation field, thereby-preventing dislodgement of the charges and collected particles from the moving surface.

In preferred embodiments, a dielectric converging-diverging nozzle located adjacent the corona. charging field increases the velocity of the charged particles to retard migration of the charged particles to the attractor electrode and to increase the potential of the space charge field built up in the collector section of the precipitator.

Preferably, the corona discharge field is established between a screen'attractor porous to the dust particles and a plurality of emitter electrodes downstream of the attractor.

This configuration produces increased precipitation efficiencyv because the turbulence, i.e., eddy currents, generated in the. gas stream by the upstream component of the corona field enhances the precipitation rate of small, submicron size particles and because the plurality of emitter electrodes provides a charging field of sufficient length to charge large, supermicron size particles to near saturation levels, thus ensuring that the precipitation rate of the'large particles will be high due to electrostatic diffusion. If desired, the turbulence can be in duced and the particles charged separately by independent devices.

In a preferred embodiment, the screen attractor is generally U-shaped, with the open end downstream, and is attached across the upstream entrance of the converging-diverging nozzle. The emitter electrodes are located centrally of the attractor and are spaced in the direction of particle flow. The screen attractor, emitter electrodes and nozzle therefore may constitute a separate assembly which, although especially adapted for use in a precipitator having moving collection surfaces of the type disclosed, is suitable for independent use in other applications.

The moving collection surface preferably is an endless belt having at least one dielectric surface and the precipitation field is established by impressing a surface charge on the dielectric surface and thereafter passing the charged dielectric surface in spaced relation to a reference electrode. A conductive substrate is located behind the moving dielectric surface to maintain a strong electric field tending to maintain the charges on the dielectric surface. The dust is removed from the moving surface by brushes which engage the surface as it passes a cleaning station that is isolated from the gas stream to avoid reentrainment of the collected dustlparticles.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be made to thefollowing description of an exemplary embodi ment, taken in conjunction with the figures of the accompany ing drawings, in which:

FIG. I is a side elevational view partly in section of one embodiment of the electrostatic precipitator of the present invention;

FIG. 2 is an expanded detail view of the moving belts and reference electrode of FIG. 1;

FIG. 3 is a side elevational view partly in section ofa second embodiment of the invention;

FIG. 4 is a horizontal sectional view taken along the line 4-4 of FIG. 3, looking in the direction of the arrows;

FIG. 5 is a side elevational view of a further embodiment of the present invention;

FIG. 6 is a horizontal sectional view taken along the line 6-6 of FIG. 5, looking in the direction ofthe arrows;

FIG. 7 is an expanded detailed view of the moving belt and a reference electrode of FIG. 6;

FIG. 8 is an expanded detailed view of the particle charging ionizer assembly of the invention; and

FIG. 9 is a sectional view taken along the line 9-9 of FIG. 8, looking in the direction of the arrows.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT In FIG. I, a representative embodiment of the invention in cludes a pair of endless belts extending in a vertical direction between opposed sets of electrically grounded rollers 22 and 24, a stationary passive electrode 26 positioned equidistant between the belts 20, brushes 28 for cleaning the belts 20 and hoppers 30 for receiving the dust from the belts.

Preferably, each belt 20 has a dielectric body portion 32 of high resistivity and a conductive backing member 34 in the form of a metallic plate or coating (see FIG. 2). The passive electrode 26 is similarly constructed and includes a metallic backing member 36 having both sides covered with a dielectric coating 38. Both the backing members 34 and 36 preferably are connected to ground, but they may be maintained at any desired reference potential.

Either or both of the rollers 22 and 24 are driven in any suitable manner to move the belts 20 past the electrode 26. The direction in which the belts are driven relative to the gas flow stream is unimportant, that is, they can travel in the same direction as, or opposite or transverse to, the direction of flow of the gas stream.

The belts 20, together with the reference electrode 26, define two gas flow channels 40 through which the dust particles are passed. Adjacent the intake ends of the channels 40, and in association with each ofthe belts 20, are disposed a plurality of high-voltage corona wires 42 for impressing surface charges on the dielectric surfaces 44 of the belts. Additional sets of corona wires 42' and 42" are disposed at predetermined intervals along the channels for purposes of recharging the belt surfaces 44 should such recharging become necessary.

As schematically illustrated in FIG. I, the corona wires 42 are connected to a power source 43 to set up the discharge between the wires and the belt surfaces 44. The recharging wires 42 and 42" preferably are connected to same source 43 as the wires 42 but may, if desired, be connected separately to individual power sources (not shown). While three wires are depicted in FIG. 1 in each set of wires 42, 42' and 42", this is for illustrative purposes only and any suitable number may be used.

The entering dust particles pass into the flow channels 40 through an ionizer assembly 46, including a convergingdiverging nozzle 48 defining a throat and located across the entrance to the channels. As will be more fully described hereinafter, the particles are given a high turbulent and/or electrostatic mobility upon passage through the ionizer assembly.

The charges applied to the belt surfaces and to the dust particles may be of either sign, it being necessary only that they be of opposite sign. This, of course, can be accomplished by selecting appropriate polarities for the ionizer 46 and the belt charging wires 42. For illustrative purposes the belt surfaces 44 and the dust particles 50 are shown in FIG. 2 as carrying positive and negative charges, respectively.

The presence of the positive charges on the dielectric surface 44 establishes a strong electric field in the dielectric member 32 between the surface 44 and the grounded backing member 34 and, in addition, sets up an electric precipitation field between the surface 44 and the passive central electrode 26 upon movement of charged portions of the belts into a position opposite the electrode 26. Because of the proximity of the backing member 34 to the charged surface 44, the electric field between them is substantially stronger than either the precipitation field or the space charge field built up in the channels 40. Consequently, the positive charges applied to the belt surfaces 44 are electrostatically held on the surfaces against the attractive force of both of those fields. The resistivity of the dielectric portion 32 of the belts, therefore, should be sufficiently high that the charges will not leak off to the backing members 34.

For optimum efficiency, the total charge initially applied to the belt surfaces 44 should be substantially greater than the total charge carried into the channels 40 by the dust particles so that residual positive charges will always remain on the belt surfaces to maintain the precipitation field and to give a net positive charge to all of the particles deposited on the belts. That is, there should always be enough residual charges on the collection surfaces to reverse the polarity of the oppositely charged particles when they are contacted with the surfaces. This is illustrated schematically in FIG. 2, where the negative ly charged particles 50 entrained in the gas stream are shown as having acquired a positive charge after precipitating on the belt surfaces 44.

As more and more particles are collected, the total charge on the surfaces 44 is reduced and, in certain circumstances, such as where extremely high dust concentrations are present in the gas streams, it sometimes is desirable to recharge the belt surfaces one or more times within the channels 40, as, for example, with the additional corona wires 42 and 42". Periodic recharging of the belt surfaces as the layer of collected particles builds up makes possible the use of lower belt speeds and higher gas velocities, and thus further enhances the efficiency of precipitation.

Because the collected particles acquire a net positive charge, they are securely held on the collection surface 40 by virtue of their attraction toward the grounded backing members 34. It can be demonstrated that so long as there are sufficient positive charges on the belt to reverse the polarity of the collected particles, the field between the particles and the backing members is significantly stronger than the electrostatic forces tending to pull them off. For example, in a typical installation where the surface charge on the belt surface 44 is 1.3 3x coulombs/mfi, the thickness of the dielectric member 32 is X 10 inches, the width of the gas flow chan- 5 nel is 0.5 inch, the strength of the precipitation field is 2 l0 volts/m. and the strength of the field between the particles and the backing member 34 is l0 volts/m., the hold ing pressure tending to retain the particles on the surface 44 is approximately 1.24 pounds/in It is apparent, therefore, that the collected particles are securely retained on the belt surfaces 44 against dislodgement, whether by the electrostatic fields in the channels 40 or by the movement of the gas stream past the collection surfaces. Thus, there is virtually no reentrainment of the dust particles in the gas stream. Moreover, on the rare occasions when a particle is reentrained, it is automatically reprecipitated due to its retention of a positive charge.

With the particles firmly held on the belts 20, they are carried out of the channels 40 and are subsequently removed from the belt surfaces 44 by the brushes 28. In this way, the collection surfaces are cleaned with minimal reentrainment of the collected particles in the gas stream resulting from the cleaning operation itself. As noted, reentrainment of the particles due to motion of the gas past the collection surfaces is also effectively prevented by the strong electric field existing between the collected particles and the backing members 34. Accordingly, gas velocities on the order of 4 times as great as those possible in conventional precipitators can be used. This is an important aspect of the invention in that it facilitates the use of a turbulence, inducing and long residence time ionizer for charging the dust particles, as will be described more fully below, and therefore contributes significantly to the overall increase in precipitator efficiency.

After cleaning, the belt surfaces are again moved past the corona wires 42, are charged as before and are again moved into collecting relation to the gas stream. Unsaturated collect ing surfaces, therefore, are continuously present to receive the dust particles exiting from the ionizer assembly 46, and the weakening influence of collected dust particles on the precipitation field, as is observed in the prior art, fixed-collection plate precipitators, is substantially reduced.

In applications where the dust being precipitated is not electrically conductive, the brushes 28 conveniently may be metal and connected to ground, as shown in FIG. 1. Where the dust particles are electrically conductive, however, either brushes constructed of dielectric material or electrically isolated brushes are preferred. Alternatively, conductive dust particles can be discharged, or neutralized immediately prior to brushing with a corona discharge current. The hoppers 30 preferably are grounded so that in any event the dust particles will be discharged upon contact with the hoppers.

Another embodiment of the precipitator of the present invention is illustrated in FIGS. 3 and 4, and includes a plurality of dielectric belts 120 of generally the same structure as the belts 20 of FIGS. 1 and 2 extending between sets of grounded drive rollers B22 and 124. The gas flow streams move horizontally through the flow channels 140 in a direction transverse to the charged belt surfaces 144, as shown by the arrows in FIG. 4. Two sets of corona wires 142 are provided to charge the individual belt surfaces 144, one of the sets being positioned adjacent each of the lower and upper rollers 122 and 124 at a point just before the belts move into the channels 140. It will be understood that if the belts I20 are driven in a different direction, the corona wires 142 would be repositioned accordingly.

A passive reference electrode I26 is interposed between adjacent belts I20 so that a precipitation field is established 7( across the gas flow channels I40. The dust-charging ionizer assemblies 146, one for every two flow channels, extend in the vertical direction across the intake ends ofthe channels.

Brushes 128 are mounted adjacent the lower rollers I22 to remove the collected particles from the belt surfaces. A common hopper 152 is located beneath the brushes I28 to catch the particles falling from the belts 120, and preferably is grounded to discharge the particles.

Preferably, the hopper 152 is connected at its upper end to a housing 154 which encloses the precipitator apparatus to form a complete unit. The housing I54 includes baffle plates 156 which define the upper and lower limits of the gas flow channels and which also serve as air locks to prevent particles dislodged from the belts by the brushes 128 from reentering the gas flow stream. If desired, the ionizer assemblies 146 can be supported from the baffle plates.

A further embodiment of the precipitator apparatus is shown in FIGS. 5 to 7. In this embodiment, a single dielectric belt 220 is passed in serpentine fashion over alternately spaced rollers 222 and 224 and defines with the passive electrodes 226 a plurality of flow channels 240. This configuration allows both sides of the belt to be used as particle collection surfaces, a feature of substantial advantage where heavy dust concentrations are present in the gas stream. Accordingly, the belt 220 constitutes a sandwich structure having two dielectric body portions 232 and' an intermediate metal-backing member 234 which is grounded or otherwise connected to a reference potential (see FIG. 7).

The reference electrodes 226 have the same structure as the electrode 26 of FIG. 1, including a metal plate 236 covered on both sides with dielectric films 238. Since the sidewalls 254' of the housing 254 act as reference electrodes for the adjacent legs of the belt 220, they preferably have a dielectric coating on their inner surfaces and are also connected to a reference potential. A set of vertically extending corona wires 242 is positioned on each side of the belt 220 adjacent each point where the belt enters the gas flow channels 240. Thus the surfaces 244 on both sides of the belt 220 are recharged for each pass through the gas stream.

The dust particles 250 are fed through the channels 240 in a vertical direction, see the arrow in FIG. 5, and are charged at the intake ends of the channels in horizontally extending ionizer assemblies 246 similar to that represented in FIG. 1. Since only one belt is used, a single pair of brushes 228 suffices to remove the collected particles from the belt surfaces 244. A hopper 252 is connected to the housing 254 in vertical alignment with the brushes, and a pair of rollers 258 rotatably carry the belt 220 in overlying relation to the hopper 252 so that particles dislodged from the belt by the brushes fall directly into the hopper. Baffle plates 256 are positioned in the housing 254 inwardly of each of the rollers 222 and 224 to confine the gas streams and to isolate the flow channels 240 from the area of the brushes 228. If the dust collected is conductive, the rollers 222, 224 and 258 supporting the belt preferably are isolated electrically from ground so that no discharge path is provided for charges on the collected particles.

In all of the above embodiments, the passive electrodes 26, 126 and 226 have been illustrated and described as being at ground potential. This is possible because a total charge of sufficient magnitude to set up a precipitation field of suitable strength can be imparted to, and retained on, the belt collection surfaces, as has been set forth heretofore. It is also advantageous in that a separate power supply need not be provided to apply a potential to the passive electrode; hence, significant cost savings are realized. If a stronger precipitation field is desired, however, a suitable power supply (not shown) can be used to raise the passive electrode to the required potential. Inthis instance, the potential applied to the passive electrode would of course be opposite in polarity to that of the belts.

Because the foregoing embodiments of the moving collec tion surfaces allow the use of increased gas velocities, they are advantageously used in combination with, and as a supplement to, a dust ionizer of a kind found useful in the precipitation of particles of widely varying sizes, that is, an ionizer which simultaneously generates turbulence in the gas stream, charges the dust particles electrically and increases the velocity of particle flow. This is of particular importance, as is made clear below, because the introduction of turbulence into the gas stream and the increase of the dust particle velocities are significant factors in obtaining efficient precipitation of the dust particles normally found in industrial exhaust gases.

Referring now to FIGS. 8 and 9 in particular, the ionizer sembly, indicated generally at 46, essentially includes a metal screen attractor electrode 300, a plurality of spaced emitter electrodes 302 and the dielectric-converging-diverging nozzle 48. Suitably, the attractor is grounded and the emitter electrodes maintained at an appropriate potential through connection with the power source 304, the precise potential applied to the emitter electrodes depending upon the spacing of the electrodes from the attractor and upon gas flow conditions. The opening size of the screen attractor 300 is also variable depending upon conditions; for example, a screen having l8 openings per inch has been used successfully.

The attractor 300 conveniently is U-shaped and has its open end facing down stream and supported by the nozzle 48 at approximately the point of minimum dimension of the nozzle, that is, at approximately the midpoint of the nozzle. Thus, the dust particles SO approaching the ionizer assembly 46 are drawn at increased velocity, due to the well known venturi phenomenon, through the porous attractor 300, past the emitter electrodes 302 and are then exited from the nozzle into the region of the precipitation field set up between the moving belts and the passive electrode 26. By virtue of the corona discharges set up between the individual emitter electrodes 302 and the surrounding screen electrode 300, an elongated corona discharge field is established across the path of and at least partly in opposition to the flow of the particles 50 as they pass through the nozzle 48. The particles 50 are therefore charged electrically, as shown schematically in FIG. 8, and turbulence is simultaneously generated in the gas stream because the gas flow through the porous attractor 300 is everywhere opposed by an electric wind" set up by the corona discharges from the wires 302 to the screen.

As noted, this particular arrangement of the ionizer is especially adapted to allow high efficiency of precipitation of dust particles of various sizes. This result is obtained because the elongation of the corona discharge field in the direction of particle flow subjects the incoming particles 50 to an extended period of ion bombardment, that is, the particles have a long residence time in the charging field, so that they are able to acquire a high electrical charge. Moreover, the eddy currents induced in the gas stream by the rearwardly directed component of the corona fields causes chaotic motion of the particles,with the result that the particles are retained longer in the charging field and attain greater charge and mobility. This is particularly important with respect to the larger supermicron size particles, as the predominant forces reacting with the larger particles to cause them to precipitate are electrostatic, namely, the electric precipitation field and the space charge field existing in the collector portion of the precipitator. It is desirable, therefore, that the larger particles attain near saturation level charges, and the lengthened residence time afforded by the elongated corona field in the ionizer 46 is effective for this purpose.

Generally, enough wires 302 should be provided in the ionizer to set up a corona field of the needed length. The actual number required is determined by the circumstances of a given application, including such factors as the size range of the particles to be collected, the velocity ofthe gas stream and the dimensions of the ionizer assembly.

With extended residence times, however, there is the problem of the charged particles being attracted to the screen 300 and becoming discharged. This is overcome in the present ionizer by the use of the converging-diverging nozzle 48 which imparts to the particles a velocity sufficiently high to oven come the attractive forces between the particles and the screen electrode. The dielectric nozzle 48, in addition, serves as an electrogasdynamic generator in that it delivers large concentrations of charged dust at high velocities to the collector portion of the precipitator, where the energy of the incoming dust is converted in the well-known manner to electrical potential energy by the resistance of the space charge field built up by the charged particles. As high space charge densities are required for optimum performance, the overall efficicncy of the precipitator is therefore increased.

Inasmuch as small submicron size particles attain an electrical charge much more slowly than do the larger particles, they are not acted upon by the electrostatic forces of the precipita tion and the space charge fields to the same extent as are the larger particles. Accordingly, the migration rate, or drift speed, due to electrostatic forces of the smaller particles toward the particle collection surfaces is correspondingly less. Conversely, it has been found that these particles are affected to a much greater extent than the larger particles by turbulence in the gas stream. It has further been discovered that by generating a controlled amount of turbulence in the gas stream by setting up a corona discharge in opposition to the gas flow, migration of small particles to the collection surfaces is substantially increased.

Of course, the small particles attain some electric charge and are acted upon to some extent by the electrostatic fields. In particular, it is thought that the electrostatic fields are effective to drive the particles across the thin laminar boundary layer that may exist at the collection surfaces. This effect is likewise thought to occur with the larger particles.

It will be apparent from the foregoing, that the precipitator of the present invention is especially adapted for a wide variety of applications, whether with light or heavy dust concentrations or small or large particle sizes, or any combination of either. Moreover, the ionizer assembly, although especially suited for use with moving collection surfaces of the kind disclosed, is also adapted for separate use. It could, for example, be used to advantage in conventional, two-stage precipitators to increase their efficiency of precipitation of small submicron size particles, or in new installations in combination with any desired form of collection surfaces. Further, the turbulence can be induced in the gas stream through a mechanism other than the rearwardly directed corona field, for example, baffle plates could be positioned adjacent the entrances to the gas flow channels in the collection section so as to interrupt the smooth flow of the stream. ln such case, the particles could be charged in a corona field established principally normal to the direction of particle flow.

It will further be apparent to those skilled in the art that the above'described embodiments are intended to be merely exemplary, in that they are susceptible of modification and variation without departing from the spirit and scope of the invention.

We claim:

1. Apparatus for electrostatically precipitating particles entrained in a gas stream comprising means defining at least one moving surface located adjacent the gas stream boundary, means for establishing a corona discharge in an upstream location in the gas stream having a substantial component in opposition to the gas stream to impart an electrical charge to the entrained particles and to introduce turbulence into the gas stream, and means for establishing an electric precipitation field normal to the moving surface and reacting with the charged particles to move them into contact with the moving surface.

2. Apparatus according to claim 1 further comprising means for establishing an electric field exteriorly of the gas stream and having a value exceeding the value of the electric precipitation field and reacting with the particles collected on the moving surface to retain them thereon.

3. Apparatus according to claim 2 in which the moving surface is constructed of dielectric material and in which the means for establishing an electric field exteriorly of the gas stream includes means for imparting to the moving dielectric surface an electric charge of opposite polarity to that imparted to the entrained particles and an electrically conductive member located closely adjacent the dielectric surface and exterior to the gas stream.

4. Apparatus according to claim 3 in which the moving dielectric surface comprises at least one endless belt having one side constructed of a dielectric material and the other side constructed of an electrically conductive material.

5. Apparatus according to claim 4 in which both sides ofthe endless belt are constructed of dielectric material and an in' termediate layer of electrically conductive material is disposed therebetween.

6. Apparatus according to claim I in which the corona discharge means includes a screen disposed to accept the gas flow, the screen being porous to the entrained particles and defining an attractor electrode, and at least one emitter electrode downstream of the attractor electrode.

7. Apparatus in accordance with claim 6 in which the corona discharge means further comprises means for retarding the migration of charged particles to the attractor electrode.

8. Apparatus according to claim 7 in which the means for retarding particle migration comprises a converging-diverging nozzle constructed ofdielectric material and located in the gas stream to accept the flow passing through the attractor electrode.

9. Apparatus according to claim 8 in which the dielectric nozzle is located at least partly intermediate the corona discharge field and the moving surface.

10. Apparatus according to claim 6 in which the porous screen is elongated in the general direction of flow of the gas stream and in which a plurality of emitter electrodes are spaced therealong to establish a plurality of corona discharges in the gas stream.

11. Apparatus in accordance with claim 10 in which the porous screen is generally U-shaped, with the open end facing downstream, and the plurality of emitter electrodes are spaced within the U-shaped attractor electrode along the general direction of flow of the gas stream.

12. Apparatus in accordance with claim 10 further comprising a converging-diverging nozzle constructed of dielectric material and located in the gas stream at least partly intermediate the downstream end of the porous screen and the moving surface.

13. Apparatus according to claim 11 further comprising a converging-diverging nozzle defining a throat and located in the gas stream and constructed ofdielectric material, the open downstream end of the screen adjoining the throat of the nozzle.

14. Apparatus for imparting an electrical charge to particles entrained in a gas stream and for introducing turbulence into the gas stream comprising a screen porous to the entrained particles located across the path of flow of the particles and defining an attractor electrode, the screen being elongated in the general direction of flow of the particles, and a plurality of emitter electrodes located downstream of a portion of the screen and spaced along the elongated length of the screen, the emitter and attractor electrodes being adapted for connection to a power supply for establishing corona discharges from the emitter electrodes to the porous screen.

15. Apparatus according to claim 14 further comprising means for retarding migration of the charged particles to the porous attractor electrode.

16. Apparatus in accordance with claim 15 in which the migration retarding means includes a converging-diverging nozzle constructed of dielectric material and located across the gas stream adjacent to the downstream end ofthe screen.

17. Apparatus in accordance with claim 14 in which the porous screen is generally U-shaped, with the open end facing downstream, and the plurality of emitter electrodes are spaced within the U-shaped attractor electrode along the general direction of flow ofthe gas stream.

18. Apparatus according to claim 17 further comprising a converging'diverging nozzle defining a throat and located in the gas stream and constructed of dielectric material, the downstream end of the U-shaped screen adjoining the nozzle near the throat thereof.

19. Apparatus for electrostatically precipitating particles entrained in a gas stream, comprising means for imparting an electrical charge to the entrained particles; means defining at least one member downstream of the particle charging means adjacent to and movable along the gas stream boundary, the surface of the member facing the gas stream being dielectric; means for imparting to the dielectric surface an electrical charge of opposite polarity to that of the charge imparted to the particles to create an attractive f-orce therebetween; means for subjecting the charges on the dielectric surface to an electrostatic force normal to the surface and reacting with the charges to retain them thereon that is greater than any corresponding electrostatic force in an opposite direction created by electrical fields within the gas stream; and means for inducing turbulence in the gas stream to create gas currents tending to carry entrained particles to the dielectric surface.

20. Apparatus according to claim 19 in which the particlecharging means comprises an attractor electrode and a corona electrode, and in which the turbulence inducing means includes means for locating the corona electrode downstream of the attractor electrode to create a corona discharge in opposition to the gas stream. 

2. Apparatus according to claim 1 further comprising means for establishing an electric field exteriorly of the gas stream and having a value exceeding the value of the electric precipitation field and reacting with the particles collected on the moving surface to retain them thereon.
 3. Apparatus according to claim 2 in which the moving surface is constructed of dielectric material and in which the means for establishing an electric field exteriorly of the gas stream includes means for imparting to the moving dielectric surface an electric charge of opposite polarity to that imparted to the entrained particles and an electrically conductive member located closely adjacent the dielectric surface and exterior to the gas stream.
 4. Apparatus according to claim 3 in which the moving dielectric surface comprises at least one endless belt having one side constructed of a dielectric material and the othEr side constructed of an electrically conductive material.
 5. Apparatus according to claim 4 in which both sides of the endless belt are constructed of dielectric material and an intermediate layer of electrically conductive material is disposed therebetween.
 6. Apparatus according to claim 1 in which the corona discharge means includes a screen disposed to accept the gas flow, the screen being porous to the entrained particles and defining an attractor electrode, and at least one emitter electrode downstream of the attractor electrode.
 7. Apparatus in accordance with claim 6 in which the corona discharge means further comprises means for retarding the migration of charged particles to the attractor electrode.
 8. Apparatus according to claim 7 in which the means for retarding particle migration comprises a converging-diverging nozzle constructed of dielectric material and located in the gas stream to accept the flow passing through the attractor electrode.
 9. Apparatus according to claim 8 in which the dielectric nozzle is located at least partly intermediate the corona discharge field and the moving surface.
 10. Apparatus according to claim 6 in which the porous screen is elongated in the general direction of flow of the gas stream and in which a plurality of emitter electrodes are spaced therealong to establish a plurality of corona discharges in the gas stream.
 11. Apparatus in accordance with claim 10 in which the porous screen is generally U-shaped, with the open end facing downstream, and the plurality of emitter electrodes are spaced within the U-shaped attractor electrode along the general direction of flow of the gas stream.
 12. Apparatus in accordance with claim 10 further comprising a converging-diverging nozzle constructed of dielectric material and located in the gas stream at least partly intermediate the downstream end of the porous screen and the moving surface.
 13. Apparatus according to claim 11 further comprising a converging-diverging nozzle defining a throat and located in the gas stream and constructed of dielectric material, the open downstream end of the screen adjoining the throat of the nozzle.
 14. Apparatus for imparting an electrical charge to particles entrained in a gas stream and for introducing turbulence into the gas stream comprising a screen porous to the entrained particles located across the path of flow of the particles and defining an attractor electrode, the screen being elongated in the general direction of flow of the particles, and a plurality of emitter electrodes located downstream of a portion of the screen and spaced along the elongated length of the screen, the emitter and attractor electrodes being adapted for connection to a power supply for establishing corona discharges from the emitter electrodes to the porous screen.
 15. Apparatus according to claim 14 further comprising means for retarding migration of the charged particles to the porous attractor electrode.
 16. Apparatus in accordance with claim 15 in which the migration retarding means includes a converging-diverging nozzle constructed of dielectric material and located across the gas stream adjacent to the downstream end of the screen.
 17. Apparatus in accordance with claim 14 in which the porous screen is generally U-shaped, with the open end facing downstream, and the plurality of emitter electrodes are spaced within the U-shaped attractor electrode along the general direction of flow of the gas stream.
 18. Apparatus according to claim 17 further comprising a converging-diverging nozzle defining a throat and located in the gas stream and constructed of dielectric material, the downstream end of the U-shaped screen adjoining the nozzle near the throat thereof.
 19. Apparatus for electrostatically precipitating particles entrained in a gas stream, comprising means for imparting an electrical charge to the entrained particles; means defining at least one member downstream of the particle charging means adjacent to And movable along the gas stream boundary, the surface of the member facing the gas stream being dielectric; means for imparting to the dielectric surface an electrical charge of opposite polarity to that of the charge imparted to the particles to create an attractive force therebetween; means for subjecting the charges on the dielectric surface to an electrostatic force normal to the surface and reacting with the charges to retain them thereon that is greater than any corresponding electrostatic force in an opposite direction created by electrical fields within the gas stream; and means for inducing turbulence in the gas stream to create gas currents tending to carry entrained particles to the dielectric surface.
 20. Apparatus according to claim 19 in which the particle-charging means comprises an attractor electrode and a corona electrode, and in which the turbulence inducing means includes means for locating the corona electrode downstream of the attractor electrode to create a corona discharge in opposition to the gas stream. 